A fuel cell is a power generator, which converts chemical energy stored within fuel and oxidants directly into electrical energy through reactions of its electrodes. The different types of fuel cells are diverse and their classifications varied. According to the properties of their electrolytes, fuel cells can be divided into five types including alkaline fuel cells, phosphoric acid fuel cells, proton exchange membrane fuel cells, fused carbonate fuel cells, and solid oxide fuel cells. Wherein, a proton exchange membrane fuel cell includes a so-called direct methanol fuel cell (DMFC), which directly uses methanol as fuels without modifying the same into hydrogen gas. This is also at present a technique that can generate relatively high power. Such fuel cells may be applied to large power plants, vehicular power generators, portable power supplies, and so forth.
It is essential to control the concentration of liquid fuels while commercializing such fuel cells as DMFC. Theoretically, fuels with lower concentrations produce less electricity, and fuels with higher concentrations produce more electricity. Accordingly, to maintain the concentration at a predetermined level, a concentration detector is needed to monitor the concentration of fuels in real-time. As such, the electricity of fuel cells is qualified, and the electronic products using the cells will not be damaged due to the unsteady power supplied by the fuel cells.